In numerically controlled machine tools, in particular milling machines, switching probe heads are often used for carrying out automated coordinate measurements. Such probe heads have a stylus, which triggers a switching event when it contacts an obstacle, such as the surface of a workpiece. Typical fields of application for automated measurements include the aligning of workpieces, setting of reference points, measuring of workpieces for quality control, and the digitizing of three-dimensional workpiece surfaces.
To carry out the measurements, the probe head is inserted into the tool spindle in place of a tool. In this manner, it may be positioned in the different coordinate axes by the numerical control unit of the machine tool. For coordinate measurement, the probe head, controlled by the numerical control unit, is moved towards the workpiece until a switching event indicates that the surface of the workpiece has been reached. This operation is referred to as a probing operation. During the probing operation, the spatial position of the probe head in or with respect to the coordinate axes to be measured is continuously measured by position-measuring devices in time intervals determined by the cycle time of the numerical control unit, and the position values are transmitted to the numerical control unit. The position values are needed by the numerical control unit as actual position values for its internal control loops. Customary cycle times are, for example, on the order of 50 μs.
Since, in this procedure, the position values are ascertained in a discrete-time manner in fixed time intervals, but the switching event occurs asynchronously to this, a measuring error results that increases with the probing speed and the cycle time. This measuring error adds to the general measuring error of the machine tool and therefore reduces the quality of the overall system. If, for example, one takes the above-mentioned cycle time of 50 μs and a probing speed of 1 m/min, then a maximum position error of approximately 0.83 μm results. That corresponds to the distance that the probe head travels in 50 μs at a probing speed of 1 m/min. In many cases, an additional position error of this magnitude is not acceptable.
Since the cycle time of numerical control units, as is conventional, cannot be influenced, reducing the probing speed is mostly the only option for reducing the additional position error produced by the discrete-time method of functioning. However, this also results in a lower throughput and, therefore, reduction of economic efficiency.
European Published Patent Application No. 0 073 495 describes a compromise between high probing speed and low position error. Described is a method for coordinate measurement, where a first probing is carried out at high probing speed, the probe head is subsequently moved a short distance away from the workpiece surface again, and the probing is repeated at a lower probing speed. A higher accuracy is attained, using the lower probing speed during the second probing operation. The disadvantage of this method is that it sets high requirements for the programming of the numerical control unit, and the time expenditure from the two-time change of direction cannot be significantly reduced.